Elastography imaging can be thought of as a form of objective medical palpation, a medical technique that is commonly used by medical doctors to diagnose disease. Through palpation, stiffer, asymmetric, significantly heterogeneous, or otherwise abnormal tissue can be felt and is often indicative of diseases—as found in liver, prostate or breast malignancies.
The quantity depicted or inferred through elastography is elasticity. Elasticity is also referred to as stiffness, or the inverse of compliance. Elastography techniques can also measure the viscoelastic properties of tissue, such as viscosity and relaxation time. In elastography, a mechanical excitation is applied in the proximity of the tissue of interest, such as prostate, breast, liver or any other soft organ in the body, and the resulting deformation is measured. The resulting deformation is typically measured with ultrasound (the method known as ultrasound elastography or USE) or Magnetic Resonance Imaging (the method known as magnetic resonance elastography or MRE), as these imaging modalities do not involve the use of ionizing radiation. The deformation is post-processed to extract information such as viscoelastic properties (e.g., shear modulus and viscosity). The relative deformation or tissue strain, or alternatively, the intrinsic mechanical properties of tissue, can be displayed as a map of stiffness (or other meaningful mechanical properties) of the imaged object.
The main advantage of MRE is that it creates high quality quantitative images of the mechanical properties of tissue based on all displacement directions, and that the MR elastography images can be registered and superimposed onto other MR modalities such as T2-weighted imaging, that are excellent descriptors of anatomy or function. The problem with MRI is that MRI is a relatively slow imaging modality, so MRE typically requires many minutes of acquisition time.
Ultrasound provides faster acquisition yet it poses other challenges to overcome due to the pulse-echo nature of data acquisition and need for multiple pulses, which introduce time delays from both time of flight of the pulses and the delays between pulses in conventional ultrasound machines. Furthermore, tissue motion, as captured by an ultrasound transducer, usually represents the motion in the axial direction with respect to the ultrasound transducer, as the resolution of an ultrasound image is generally highest in the axial direction and lowest in the elevational direction, so tissue motion in the axial direction is measured with the highest accuracy. The other directions—lateral and elevational—are not as accurate. Ultrasound has a limited field of view and is not able to easily map the full abdominal cavity. Furthermore, the tissue contrast provided by standard ultrasound images, while also registered to elasticity images, is much poorer than for MRI.